The Resonator Fiber Optic Gyroscope (RFOG) has the potential of meeting the needs in many areas of the inertial sensing market. To overcome optical backscatter errors, currently available RFOGs lock the clockwise (CW) and counter-clockwise (CCW) laser frequencies onto different longitudinal modes of the gyro sensing resonator. These technologies separate the counter-propagating laser frequencies and up-convert the interference signals between counter-propagating lightwaves well above the rotation measurement frequency band. However, if only two lasers are used, the gyro sensing resonator free spectral range (FSR) becomes a part the rotation measurement. Thus, in prior art RFOGs, the gyro sensing resonator free spectral range must be measured with great precision to reduce the adverse effects caused by FSR variations and more than two lasers are typically used.
To avoid interference between the CW and CCW light, which happens due to back reflection when lasers are locked to the same resonance at low rotation rate, the prior art uses methods of locking three or more lasers to three different resonances. To accurately measure the FSR, currently available RFOGs use a third laser frequency to probe the sensing resonator. Resonance frequencies of the gyro resonator must be precisely measured to obtain an accurate measure of the FSR and rotation. Modulation is used for detecting the resonance frequency. For a master/two slave configuration, it is difficult to modulate all three lasers to detect resonance in a way that modulation imperfections do not cause large errors. Slave lasers can be modulated with high precision by using digital techniques combined with high gain optical phase lock loops, but it is difficult to modulate the master laser with high precision since the modulation eventually becomes an open loop analog current or voltage.
Other currently available RFOGs use a master laser and three slave lasers. In this latter technology, the master laser is not used for rotation sensing and the three slave lasers are modulated with high precision. However, the number of lasers and associated phase lock loop electronics results in a significant increase in the cost, size, weight, and electrical power consumption of the RFOG.
Thus, an RFOG that requires only two lasers for rate measurement without errors when the FSR of the gyro sensing resonator changes (due to temperature fluctuations and the like) is preferred.